1. Field of the Invention
This invention relates to the rapid deposition of coatings on substrates by chemical vapor deposition using low vapor pressure reactants and, more specifically, to the use of powder feeding of the low vapor pressure reactants into the chemical vapor deposition furnace.
2. Prior Art
Current techniques in chemical vapor deposition ("CVD") which use low vapor pressure reagents introduce the reactant material too slowly into the reactor, which reduces the coating deposition rate. Common solutions to this problem of reduced coating deposition rate include: running the vaporizer or reactor at reduced pressures, which increases the transport rate of the reagent into the reactor; running the reactor with a high carrier gas flow rate, which tends to dilute the reactant gas stream since saturation may not be achieved; or heating the solid or liquid reactants to a high temperature, which increases the vapor pressure above the reactant.
Current coating techniques usually do not include the powder feeding or liquid feeding of low vapor pressure reactants to increase the deposition rate. One introduction technique consists of a process where reagents which are dissolved to form a liquid solution are then sprayed into a furnace to form a powder of superconducting oxides. One coating technique consists of a process where substrates are coated with superconducting oxides by spraying the substrates with or dipping the substrates in liquid reactants. These, however, are not CVD processes and do not result in an atom-by-atom build-up of the coating. These coatings, therefore, do not possess the high degree of crystalline perfection found in CVD coatings.
Previous CVD techniques have used an auger to feed ZrCl.sub.4 powder into a CVD furnace as an alternative to the use of a vaporizer. See Hollabough, C. M., L. A. Haham, R. D. Reiswig, R. W. White, and P. Wagner, Chemical Vapor Deposition Of ZrC Made By Reactions Of ZrCl.sub.4 With CH.sub.4 And With C.sub.3 H.sub.6, 35, (9), NUCL. TECH., 527-35 (1977). This approach used powder feeding of ZrCl.sub.4 as a reagent to CVD furnaces instead of using a vaporizer. However, the powder feeding method was used in place of a vaporizer because, depending on the CVD furnace design, it is sometimes difficult to keep the portion of the reagent gas line connecting a vaporizer with a CVD furnace at a sufficiently high temperature. This approach was used not because the vapor pressure of the reactant, ZrCl.sub.4, is low or because heating of ZrCl.sub.4 renders it unstable, but because a relatively high temperature is required to develop the desired vapor pressure and maintaining the high temperature throughout the gas lines going to the furnace often is difficult.
Newkirt et al. U.S. Pat. No. 4,202,931 dicloses the use of a powder feeder to prepare a Nb.sub.3 Ge low temperature superconductor. However, use of a powder feeder was known prior to this patent, as disclosed in Newkirk et al. U.S. Pat. No. 4,054,686. See also the paper by Hollabough et al. referenced above. Newkirk et al. U.S. Pat. No. 4,202,931, disclosed the use of a powder feeder to improve the control or accuracy of NbCl.sub.5 dispensing.
None of the prior art techniques have been applied to the powder feeding of the new generation of high temperature superconducting oxides, into CVD furnaces, nor have they been designed to achieve the high deposition rates desired for superconductors and other materials. The prior art techniques are suitable for the deposition of Nb and Zr compounds, as high deposition rates for these compounds can be achieved without powder feeding, for example, by vaporization of NbCl.sub.5 and ZrCl.sub.4. Vaporization cannot yield the high deposition rates for oxide superconductors, because oxide superconductor reagents have very low vapor pressures and cannot be practically (i.e., rapidly) introduced into a CVD furnace using vaporization techniques.
The vapor pressure data of FIG. 7 permits clarification of this point. The graph is a compilation of reagent vapor pressures obtained from the literature. Curves 1-8 are for yttrium, barium, and copper reagents used for the CVD of YBa.sub.2 Cu.sub.3 O.sub.x which are considered low vapor pressure reagents. Curves 9-14 are for other reagents which commonly are used for CVD of other materials which are considered high vapor pressure reagents. Two points are of interest; first, the vapor pressures of the yttrium, barium, and copper reagents are very low compared to the typical CVD reagents; second, the large discrepancy in vapor pressure of yttrium tetramethylhepatanedionate from the four literature sources (curves 2, 5, 7, and 8) is indicative of the difficulty in developing a controllable, repeatable CVD process which relies on sublimation of the beta-diketonates in vaporizers. Our experience, and that of others, has been that the vapor pressure, and therefore flowrate of reagent from a vaporizer, varies with time and from lot-to-lot of reagent. The strong variation of vapor pressure with temperature, even for a given source, is evident from FIG. 7. This necessitates very close control of vaporizer temperature if a controllable flowrate of reagent is to be achieved. Three such vaporizers are involved in the conventional CVD of YBa.sub.2 Cu.sub.3 O.sub.x.
Curves 10 and 11 in FIG. 7 are for ZrCl.sub.4 and NbCl.sub.5. These reagents have sufficiently high vapor pressures, e.g. 100 torr, within the temperature range over which they are stable and thus can be supplied by use of either vaporizers or by powder feeding. However, FIG. 7 shows that the vapor pressures of the yttrium, barium, and copper reagents (Curves 1-8) do not exceed about 0.1 torr. If these latter reagents are heated to higher temperatures than those shown in FIG. 7 in an attempt to obtain higher vapor pressures, these reagents decompose rapidly to form non-volatile compounds.
Thus, when the reagents ZrCl.sub.4 and NbCl.sub.5 were introduced via powder feeding in the prior art, it was merely at the discretion of the investigator as a matter of convenience and as an attempt to improve the accuracy of reagent flowrate, rather than for the purpose of increasing deposition rate. As ZrCl.sub.4 is not a superconductor reagent, it does not have a relevant T.sub.c, and the T.sub.c of Nb.sub.3 Ge is about 20K, which is much lower than the T.sub.c of about 84K of the superconductor materials used in this invention. However, in the case of the very low vapor pressure reagents, the use of powder feeding is mandatory in order to achieve high deposition rates, and also maintains the control over reagent flowrate (i.e. process repeatability) much more so than in the case of high vapor pressure reagents such as ZrCl.sub.4 and NbCl.sub.5 where vapor pressures can be controlled easily and are highly reproducible.
Chemical vapor deposition is used extensively for the commercial preparation of numerous electronic, optical, tribological, and chemically protective coatings. Compared to other coating processes, CVD is often faster, yields higher quality, more adherent films, and can be used to coat multiple, irregularly shaped substrates. Films of the high T.sub.c oxide superconductor YBaYBa.sub.2 Cu.sub.3 O.sub.x have been prepared by CVD. Typical reagents used in CVD processes for the oxide superconductors have consisted of metal complexes of various beta-diketonate ligands, which are solids at room temperature and which slowly sublime when heated to about 100.degree.-300.degree. C.; the vapor is then swept into the CVD system by a carrier gas. Without exception, however, the very low vapor pressure of the Y, Ba, and Cu precursor reagents has restricted deposition rates to low values, e.g. 1 micron per hour. Furthermore, the vapor pressures of the reagents are strongly dependent on temperature and are subject to change as a result of thermal-environmental induced degradation during sublimation. Hence, process control and repeatability are unusually poor.